(follow their instructions regarding Umlauts)

LEO

dict.cc - includes buttons for inserting Umlauts.

canoo.net
Feed »Canoo« a German word and you can find synonyms of the word and see example sentences using the word.
Click on Inflection and you can see info such as the gender (der/die/das) and the plural form if it's a noun.
If it's an adjective you can click on Inflection and check its comparison forms (eg gut...besser...am besten).
Click on LEO (D-E) to feed it to the LEO dictionary to find out its meaning [opens in a new window].